The production of composite articles requires the use of pressure onto the blank for forming the blank against the forming surface sections. The elastic forming medium, such as a vacuum bag, is used to achieve said pressure such that wrinkles and trapped air in the blank can be forced out under vacuum pressure. The elastic forming medium can be used for curing the resin of the blank in an autoclave or in an oven at elevated temperature and pressure. After curing, the elastic forming medium is removed from the forming tool. The blank can be made of a so called “prepreg” material (layers or plies of fibre material previously impregnated with resin, such as thermosetting resin).
It is desirable that the formation of wrinkles, at the location of bevels or other projections of the article, is eliminated in the blank during the forming process.
One way to eliminate such wrinkles is described in US 2004/0115299 A1disclosing a reusable vacuum bag. Herein is described that the vacuum bag can be made thinner within the area of a forming surface having small male or female radii to allow the reusable vacuum bag to more readily stretch and conform the blank to the forming surface.
The applicant of the present patent application has earlier filed a European patent application, not yet published, disclosing a method of conforming a blank onto a forming tool's forming surface sections by drawing vacuum against an elastic vacuum bag for transmitting forming forces to the blank. The elastic vacuum bag comprises permanently at least one elongated bag section having a different stiffness than other bag sections, wherein the location of the elongated bag section corresponds, when the elastic vacuum bag after the completion of the forming lies against the forming surface sections having the blank in between, with an elongated forming surface section of the forming tool where the blank requires a forming force being larger than that of the surrounding bag sections.
This earlier method described by the applicant is working satisfactory for directing the shearing process between the plies (or within the blank material) when formed over the forming tool.
However, there is still a need for controlling the shearing process and the forces onto the blank during the forming process per se in a flexible manner. There is also a need for providing an apparatus for conforming a blank onto a forming tool, wherein the apparatus is designed to exert variable forces and directions onto the blank depending upon the actual required forming forces to the blank must have for a specific phase during the forming process. There is also a need for providing one individual elastic forming medium that can be used for different forming tools having various forming surface sections.
There exists thus still a need to control the shearing process during the actual forming procedure between the plies of the blank being formed over the forming tool, for eliminating the risk that wrinkles appear within the plies or in the blank at the location of a bevel or other projections of the article. Other projections can be a step wise thickening of the article, wherein the steps project towards the forming surface keeping the outer surface even.
The object of the present invention is to overcome the drawbacks of known techniques and to develop the method and apparatus being disclosed in said earlier application of the present applicant.